Consumers are increasingly turning to the Internet to obtain audio content, such as music. Instead of buying music on CDs or tapes from stores, consumers download music from on-line music vendors. The music is often stored in digital form in a database of a server connected to the network. This enables users of client playback devices, such iPods to “download” copies of the music. It is desired to obtain information identifying audio content in such a way that privacy of the consumer and music vendor is preserved.